In recent years, the use of positive temperature coefficient (PTC) materials in connection with electric heating elements has become more and more widespread because of the advantages flowing from its self-limiting temperature characteristics. Various types of heating cables have been devised in which a pair of spaced conductors are separated by the PTC material which has been extruded over the two conductors. The PTC material between the two conductors provides a relatively low resistance path from the one conductor to the other. The heat is actually produced in the PTC material and one might consider the PTC material between the two conductor wires as a plurality of parallel resistance heaters disposed between the two conductor wires.
The PTC material typically may consist of a polyethylene material to which a dispersion of conductive carbon black has been added. Reference may be had to Bedard et al U.S. Pat. No. 3,858,144 which describes the constituents of suitable PTC materials for use in heating cables and the manner in which such materials might be processed including the annealing step. The Bedard et al patent also mentions the problems associated with the low contact resistance between the PTC material and the conductors which supply the current to the PTC material.
In the above-cited Bedard et al patent, improved stability and better and more consistent interface conductivity between the conductor wire and the PTC material is said to have been achieved by precoating the electrodes with a composition containing conductive black prior to deposition of the carbon black polymeric matrix by extrusion onto the electrodes. This results in the polymeric matrix having substantially greater amounts of carbon black at the interface than elsewhere. However, the type of conductor disclosed in the above-cited Bedard et al patent would not be suitable for use in electric blankets and other environments wherein repeated flexing is required and the wire must be easily flexed.